(1) Field of the Invention
The present invention relates to Schottky diodes using a nitride semiconductor material and fabrication methods for the same.
(2) Description of Related Art
Schottky diodes are basic electronic devices and used for various circuits, such as rectifier circuits, inverter circuits and chopper circuits. The electrical characteristics demanded of Schottky diodes include a high breakdown voltage. With a reduction in the size of power supply circuits and an increase in the capacitance thereof, higher-breakdown-voltage Schottky diodes have been demanded.
Disclosed as a method for enhancing the breakdown voltage of a Schottky diode is a method in which, for example, when a Schottky electrode is formed on an n-type semiconductor layer, a p-type region is formed in a region of an n-type semiconductor layer coming into contact with the lower end of the Schottky electrode by ion implantation (see, for example, A. Itoh et al. IEEE ELECTRON DEVICE LETTERS, Vol. 17, No. 3 (1996) pages 139-141). This structure can reduce the field crowding at the lower end of the Schottky electrode, resulting in an enhanced breakdown voltage.
Meanwhile, Group III-V nitride semiconductor materials composed of a compound of aluminum (Al), boron (B), gallium (Ga), or indium (In) and nitride (N) represented by a general formula of BwAlxGayInzN (wherein w+x+y+z=1 and 0≦w, x, y, z≦1) have a large bandgap. Therefore, the nitride semiconductor materials are suitable as materials of high-breakdown-voltage elements. It is expected that the use of a nitride semiconductor material for Schottky diodes will provide higher-breakdown-voltage Schottky diodes.
However, if an attempt is made to use a nitride semiconductor material for a Schottky diode of a known structure, process steps for fabricating such a Schottky diode will become complicated and the fabrication of such a Schottky diode will cost high. To be specific, in order to provide a known Schottky diode, ions must be implanted into a nitride semiconductor layer to form a P-type region. This necessitates complicated process steps, such as formation of a mask, ion implantation, removal of a resist, and annealing. In order to allow Schottky diodes to have the same characteristics, the depth to which ions are implanted into the nitride semiconductor layer must be accurately controlled.